Soils, Fertilization, and Plant Nutrition
What this domain tests
This domain tests Florida soil behavior (sand, low organic matter, low cation exchange capacity, rapid leaching), the state fertilizer rule and local ordinances, soil testing, and the arithmetic of applying nutrients at labeled rates.
Florida soils
- Most Florida upland soils are deep sands such as Myakka fine sand, with less than 2% organic matter and cation exchange capacity of only a few cmol/kg, so nitrogen, potassium, and water move down and out quickly.
- pH typically runs 5.5-6.5, but shell, limerock, and dredge fill push it above 7, tying up iron and manganese and causing interveinal chlorosis in palms, oaks, and ixora.
- Compaction and fill limit rooting; 6-8 inches of suitable topsoil or mechanical decompaction matters more than any fertilizer program.
Fertilizer rules
- The Urban Turf Fertilizer Rule (Rule 5E-1.003, F.A.C.) caps turf products at 0.25 lb P2O5 per 1,000 sq ft per application and 0.5 lb P2O5 per 1,000 sq ft per year, and requires at least 50% of the nitrogen to be slow release.
- Many counties add a 10-ft fertilizer-free zone from water bodies and impervious surfaces, deflector shields, and a summer blackout period; local rules can be stricter than state rules.
- Soil test before adding phosphorus; established Florida lawns often already test high.
- Keep single nitrogen applications near 1 lb N per 1,000 sq ft and stop nitrogen in late fall.
Worked example
A 4,000 sq ft St. Augustine lawn needs 1.0 lb N per 1,000 sq ft from a 16-4-8 product. Total N needed = 4 x 1.0 = 4 lb. Product = 4 / 0.16 = 25 lb. That 25 lb delivers 25 x 0.04 = 1.0 lb P2O5, or 0.25 lb per 1,000 sq ft, exactly at the per-application cap and half the annual cap, so any second phosphorus application that year breaks the rule. Check the label for the slow-release fraction before scheduling.
Traps
- Equating N-P-K numbers with elemental phosphorus and potassium.
- Calculating product weight instead of nutrient weight.
- Adding phosphorus without a soil test.
- Applying before rain or onto wet foliage.
- Ignoring the slow-release percentage; a fully soluble product at the same N rate leaches and can burn.